An access control list with respect to a computer file system, is a list of permissions

File system permissions

Most current file systems have methods of administering permissions or access rights to specific users and groups of users. These systems control the ability of the users to view or make changes to the contents of the filesystem....

 attached to an object

Computer file

A computer file is a block of arbitrary information, or resource for storing information, which is available to a computer program and is usually based on some kind of durable storage. A file is durable in the sense that it remains available for programs to use after the current program has finished...

. An ACL specifies which users or system processes are granted access to objects, as well as what operations are allowed on given objects. Each entry in a typical ACL specifies a subject and an operation. For instance, if a file has an ACL that contains (Alice, delete), this would give Alice permission to delete the file.
When a subject requests an operation on an object in an ACL-based security model the operating system first checks the ACL for an applicable entry to decide whether the requested operation is authorized.